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CVE 471 - 2 Reservoirs(1)

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CVE 471
WATER RESOURCES ENGINEERING
RESERVOIRS
Assist. Prof. Dr. Bertuğ Akıntuğ
Civil Engineering Program
Middle East Technical University
Northern Cyprus Campus
CVE 471 Water Resources Engineering
1/39
2. RESERVOIRS
Overview
„
„
„
Introduction
Characteristics of Reservoirs
Determination of Reservoir Capacity
„
Mass Curve Analysis
„
„
„
„
„
„
Determination of reservoir capacity for a known yield
Determination of yield from a known reservoir capacity
Sequent Peak Analysis
Operation Study
Other Approaches in Capacity Determination
Reservoir Sedimentation
CVE 471 Water Resources Engineering
2/39
2. RESERVOIRS
Introduction
„
„
„
„
„
Reservoirs are structures that store water.
In general, we observe high flow in winter and low flow in
summer, and very high values in spring months or
snowmelt seasons in Northern Hemisphere.
On the other hand, the water demand is high in summer
and low in winter.
Therefore, the regulation of the streamflow is required to
meet the demands.
This regulation is possible by constructing reservoirs in
the stream.
CVE 471 Water Resources Engineering
3/39
2. RESERVOIRS
Introduction
(continued)
• When the total supply of water (ΣS) is sufficient to meet the total
demand (ΣD) during a specified period of time, the water storage is
required when S<D.
Required Storage
S, D
S
S<D
S>D
S>D
Variation of Supply and Demand
CVE 471 Water Resources Engineering
Storage reservoir
D1
Storage reservoir
or
Diversion weir
D2
t
4/39
2. RESERVOIRS
Introduction
(continued)
„
There are number of purposes of constructing reservoirs
„
„
„
„
„
„
„
„
Irrigation,
Sediment accumulation,
Transportation,
Electricity generation,
Water supply (municipal and industrial)
Flood control, and
Recreational.
They are also used to supply emergency water like fire
fighting or stabilize pressures in the network.
CVE 471 Water Resources Engineering
5/39
2. RESERVOIRS
Introduction
(continued)
„
Reservoirs can be divided into two main categories
according to their storage capacities:
„
„
„
Storage (conservation) reservoirs
Distribution (service) reservoirs
Formation of a big reservoir behind a dam may have
various environmental aspects.
„
For example:
„
For the construction of Keban Dam in Turkey,
„
„
„
„
≈30,000 people were dislodged, and
300 km road,
48 km railway, and
A large area were expropriated.
CVE 471 Water Resources Engineering
6/39
2. RESERVOIRS
Overview
„
„
„
Introduction
Characteristics of Reservoirs
Determination of Reservoir Capacity
„
Mass Curve Analysis
„
„
„
„
„
„
Determination of reservoir capacity for a known yield
Determination of yield from a known reservoir capacity
Sequent Peak Analysis
Operation Study
Other Approaches in Capacity Determination
Reservoir Sedimentation
CVE 471 Water Resources Engineering
7/39
2. RESERVOIRS
Characteristics of Reservoirs
Potential for a Hydropower Reservoir
CVE 471 Water Resources Engineering
8/39
2. RESERVOIRS
Characteristics of Reservoirs(continued)
CVE 471 Water Resources Engineering
9/39
2. RESERVOIRS
Characteristics of Reservoirs(continued)
CVE 471 Water Resources Engineering
10/39
2. RESERVOIRS
Characteristics of Reservoirs(continued)
„
„
„
The capacity of the reservoir is very important since the
main function of the reservoir is the storage of the water.
The desired yield, which is the amount of water delivered
from a reservoir to meet downstream requirements, must
be available most of the time.
For an ideal reservoir site:
„
„
Sufficient impervious and sound formation (bed and side)
Deep and narrow valley (less evaporation, low cost of
expropriation)
CVE 471 Water Resources Engineering
11/39
2. RESERVOIRS
Characteristics of Reservoirs(continued)
„
After the determination of location of a reservoir, a
special topographic map (1:5000 scaled) of the
reservoir area is obtained with suitable contour intervals
and
„
„
Elevation - Area Curve, and
Elevation – Storage Curve are obtained.
CVE 471 Water Resources Engineering
12/39
2. RESERVOIRS
Characteristics of Reservoirs(continued)
CVE 471 Water Resources Engineering
13/39
2. RESERVOIRS
Characteristics of Reservoirs(continued)
Elevation – Area – Volume Curves for a reservoir
CVE 471 Water Resources Engineering
14/39
2. RESERVOIRS
Characteristics of Reservoirs(continued)
Storage characteristics of a reservoir
CVE 471 Water Resources Engineering
15/39
2. RESERVOIRS
Characteristics of Reservoirs(continued)
Hydropower Reservoir
CVE 471 Water Resources Engineering
16/39
2. RESERVOIRS
Overview
„
„
„
Introduction
Characteristics of Reservoirs
Determination of Reservoir Capacity
„
Mass Curve Analysis
„
„
„
„
„
„
Determination of reservoir capacity for a known yield
Determination of yield from a known reservoir capacity
Sequent Peak Analysis
Operation Study
Other Approaches in Capacity Determination
Reservoir Sedimentation
CVE 471 Water Resources Engineering
17/39
2. RESERVOIRS
Determination of Reservoirs
Capacity
„
„
The yield of a reservoir is defined as the quantity of
water, which is supplied for a certain duration.
The duration may change with the purpose of the
reservoir.
„
„
„
„
„
A few years Æ Large reservoirs
Week
Day
Hour
The yield is a function of
„
„
the inflow, and
the capacity of the reservoir.
CVE 471 Water Resources Engineering
18/39
2. RESERVOIRS
Determination of Reservoirs
Capacity
(continued)
„
„
„
„
„
Safe Yield (Firm Yield): The amount of water that is
supplied for a critical period. It is a guarantied amount
during this critical period.
Critical Period: The duration of lowest flow observed in
the records of the stream.
Depending upon the length of the flow critical period may
change so the safe yield.
Yield is not calculated certainly. The probability must be
used.
During the periods of high flow there will be extra
available water, more than the safe yield which is called
secondary yield.
CVE 471 Water Resources Engineering
19/39
2. RESERVOIRS
Determination of Reservoirs
Capacity
(continued)
„
„
„
„
Average Yield: The arithmetic average of the safe and
secondary yields over a long period.
There is a risk involved for a reservoir.
The amount of risk depends on the purpose of the
reservoir.
Target Yield: The yield determined based on the
estimated demands for a reservoir.
CVE 471 Water Resources Engineering
20/39
2. RESERVOIRS
Determination of Reservoirs
Capacity
(continued)
„
„
„
For the determination of reservoir capacity, the critical
period must be determined first.
A long period of observed flow is required.
When short period of observed flows or no observations
area available stochastic methods are used to generate
synthetic flows that has the same statistical properties
such as mean, variance, correlations etc.
CVE 471 Water Resources Engineering
21/39
2. RESERVOIRS
Determination of Reservoirs
Capacity
(continued)
„
There are four different methods to determine the
capacity if a reservoir.
„
„
„
„
Mass Curve (Ripple diagram) Method,
Sequent Peak Algorithm,
Operation Study, and
Other Approaches (Stochastic Methods and Optimization
Analysis etc…).
CVE 471 Water Resources Engineering
22/39
2. RESERVOIRS
Determination of Reservoirs
Capacity
(continued)
1. Mass Curve Analysis
(Ripple Diagram
Method, 1883)
„ One of the most widely
used methods.
„ Assumptions:
„
„
Demand is constant,
and,
the year repeats itself
continuously.
CVE 471 Water Resources Engineering
ΣS,ΣD
ΣS
ΣD
VOLUME
23/39
2. RESERVOIRS
Determination of Reservoirs
Capacity
(continued)
1. Mass Curve Analysis (Ripple Diagram Method, 1883)
If the flow is the daily or
monthly discharge then
the area under the curve
up to a certain time will
be the volume of runoff
for that period.
The slope of the mass
curve at a certain time
gives the discharge at
that time on the
hydrograph
CVE 471 Water Resources Engineering
24/39
2. RESERVOIRS
Determination of Reservoirs
Capacity
(continued)
1. Mass Curve Analysis (Ripple Diagram Method, 1883)
Critical Period
CVE 471 Water Resources Engineering
Required storage capacity of
the reservoir is the vertical
difference a+b
25/39
2. RESERVOIRS
Determination of Reservoirs
Capacity
(continued)
1. Mass Curve Analysis (Ripple Diagram Method, 1883)
a. Determination of capacity for a known yield
1. The tangents, which are
parallel to the demand line,
are plotted at the high points
(D and E).
2. The maximum departures
from the tangents to the
following low points of the
mass curve ( F and G)
determine the necessary
storage amounts V1 and V2.
The reservoir would be full at points D, D’, E, and E’.
The reservoir would be empty at points F and G.
CVE 471 Water Resources Engineering
3. The largest one of the
volumes will give the
required capacity of the
reservoir.
26/39
2. RESERVOIRS
Determination of Reservoirs
Capacity
(continued)
1. Mass Curve Analysis (Ripple Diagram Method, 1883)
b. Determination of yield for a known capacity
1. The value V of known
reservoir capacity is placed
vertically in all the low points in
the mass curve and tangents
are drawn to the previous high
points.
2. The slope of these tangents
(D1 and D2) indicate the yields
that can be supplied for those
critical periods with this given
capacity.
The plotted tangents must cut the mass curve when
extended forward, as it is the case here with points
C’ and E’. Otherwise, the reservoir will not refill.
CVE 471 Water Resources Engineering
3. The smallest one of the yields
can be supplied all the time.
27/39
2. RESERVOIRS
Determination of Reservoirs
Capacity
(continued)
„
„
„
The mass curve gives results if ΣD < ΣS during the period
of record.
The graphical approach is quite satisfactory if the
reservoir releases are constant during the period of
analysis.
When reservoir releases vary, the sequent-peak analysis
is recommended.
CVE 471 Water Resources Engineering
28/39
2. RESERVOIRS
Determination of Reservoirs
Capacity
(continued)
2. Sequent Peak Analysis
Sequent Peak Analysis is more suitable when the data of long observation
periods or long generated data are used, or when the demand is not constant.
1. Differences between inflows (S) and
demands (D) are calculated and their
summations obtained.
2. Σ(S-D) values are plotted against
time as shown in the figure.
3. On this plot the first peak value and
next larger peak (sequent peak) are
determined.
5. This process is repeated for all the
4. The storage required between these two
points is the difference between the first peak peaks in the record period as shown in
the figure also. The maximum of the
and the lowest point in this period.
storage values is the required capacity.
CVE 471 Water Resources Engineering
29/39
2. RESERVOIRS
Determination of Reservoirs
Capacity
(continued)
2. Sequent Peak Analysis
„
„
„
If the record period or generated data sequence is very long, the graphical
solution may be time consuming.
In that case and analytical solution procedure may be applied for the
analysis and it can be solved easily using a computer.
In this way, the required storage Vt at the end of a period t can be expressed
as:
D − St + Vt −1 if positive
Vt =  t
otherwise
0
„
„
At the beginning of the analysis, initially Vt-1 is set to zero and calculations
continue to find Vt values for up to twice the length of the record period.
The maximum of all the calculated values of Vt is the required storage
capacity.
CVE 471 Water Resources Engineering
30/39
2. RESERVOIRS
Determination of Reservoirs
Capacity
(continued)
3. Operation Study
„
„
„
„
The reservoir storage is considered as adequate when the
reservoir can supply all types of demands under possible losses
like seepage and evaporation.
In order to increase the operational performance of a reservoir,
the evaporation and seepage must be controlled.
The operation of a storage reservoir is also governed by the
inflow.
Rule curves indicating temporal storage requirements according
to local conditions and project demands need to be used for
effective operation purposes.
CVE 471 Water Resources Engineering
31/39
2. RESERVOIRS
Determination of Reservoirs
Capacity
(continued)
3. Operation Study
„
Rules:
„ During normal periods of river flow, the reservoir will be
maintained at the normal pool level.
„ If extremely high flows are expected, the normal pool level
can be drawn to such an elevation that the maximum
expected flood flow will be sufficient to restore the active
storage to its maximum level.
„ In the case of higher flows induced by snowmelt, there is
often enough time to empty the full content of the active pool
prior to the arrival of the flood flows.
CVE 471 Water Resources Engineering
32/39
2. RESERVOIRS
Determination of Reservoirs
Capacity
(continued)
3. Operation Study
„
The operation study is based on the solution of the continuity
equation.
dV
= I −Q
dt
where dV: differential storage during time dt
I: instantaneous total inflow
Q: instantaneous total outflow
„
An operation study may be carried out only for a period of
extremely low flows (critical period) or entire period of record.
CVE 471 Water Resources Engineering
33/39
2. RESERVOIRS
Determination of Reservoirs
Capacity
(continued)
3. Operation Study
„
Since the information concerned the time variation of inflow and
outflow is normally limited, then long term (e.g. one month)
averaged quantities of inflow and outflow are considered in
practice:
∆V
= I −Q
∆t
where ∆V: the change in storage during time interval ∆t.
I : the average inflow (runoff, precipitation etc…) during ∆t
Q : the average outflow (evaporation, seepage, controlled
outflows, mandatory releases, uncontrolled spills etc…)
during ∆t.
CVE 471 Water Resources Engineering
34/39
2. RESERVOIRS
Overview
„
„
„
Introduction
Characteristics of Reservoirs
Determination of Reservoir Capacity
„
Mass Curve Analysis
„
„
„
„
„
„
Determination of reservoir capacity for a known yield
Determination of yield from a known reservoir capacity
Sequent Peak Analysis
Operation Study
Other Approaches in Capacity Determination
Reservoir Sedimentation
CVE 471 Water Resources Engineering
35/39
2. RESERVOIRS
Reservoir Sedimentation
„
„
„
„
„
„
River carry some suspended sediment and move the larger solids
along the stream bed (bed load).
Suspended sediment and bed load are deposited as a delta at the
head of the reservoir.
Reservoir sedimentation directly affects the lifetime of a dam.
However, mechanics of reservoir sedimentation is highly
complicated.
Therefore, in practice, sediment load measurement are carried out
in streams and calibration curve (stream discharge, Q – suspened
load, Qs) is developed.
The United States Bureau of Reclamation (USBR) recommends a
sampling period of at least 5 years.
CVE 471 Water Resources Engineering
36/39
2. RESERVOIRS
Reservoir Sedimentation
(continued)
„
By examining the basin data of 16 different dams in Turkey,
Göğüş and Yalçınkaya (1992) recommended the following bestfit equation for the annual sediment yield having a correlation
coefficient of 0.89:
Y = 1906.26 Ad0.953
where
Y: the mean annual sediment yield (m3).
Ad : the drainage area (km2).
CVE 471 Water Resources Engineering
37/39
2. RESERVOIRS
Reservoir Sedimentation
(continued)
„
„
„
„
Trap Efficiency: The percentage of the inflowing sediment which is
retained in a reservoir.
Trap efficiency is a function of the ratio of the reservoir capacity to
total inflow.
The trap efficiency of a reservoir decreases with age as the
reservoir capacity is reduced by sediment accumulation.
Actually, it is very difficult to predict the sediment accumulation
„
„
almost zero accumulation during low flows
extremely large accumulation during floods
CVE 471 Water Resources Engineering
38/39
2. RESERVOIRS
Reservoir Sedimentation
(continued)
„
„
In order to increase the lifetime of a reservoir, the rate and nature
of sediment inflow to a reservoir can be controlled to a certain
extend.
This may be accomplished by:
„
„
„
„
„
„
using upstream sedimentation basins,
providing vegetative screens,
soil conservation methods, such as terraces, strip cropping, etc.
use of upstream by-pass channels,
design sluice gates at various levels to discharge suspended sediment,
Irrespective of how well the above measures are taken, all
reservoirs are eventually filled with sediment. However, the filling
period may be extended.
CVE 471 Water Resources Engineering
39/39
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